Chap. X.] GROUPS OF ALLIED SPECIES. 81 



was a Chthamalus, a very common, large, and ubi- 

 quitous genus, of which not one species has as yet 

 been found even in any tertiary stratum. Still more 

 recently, a Pyrgoma, a member of a distinct sub-family 

 of sessile ciiTipedes, has been discovered by Mr. Wood- 

 ward in the upper challv ; so that we now have abun- 

 dant evidence of the existence of this group of animals 

 during the secondary period. 



The case most frequently insisted on by palaeonto- 

 logists of the apparently sudden appearance of a whole 

 group of species, is that of the teleoste^n fishes, low 

 down, according to Agassiz, in the Chalk period. This 

 group includes the large majority of existing species. 

 But certain Jurassic and Triassic forms are now com- 

 monly admitted to be teleostean ; and even some 

 palaeozoic forms have thus been classed by one high 

 authority. If the teleosteans had really appeared 

 suddenly in the northern hemisphere at the commence- 

 ment of the chalk formation the fact would have been 

 highly remarkable ; but it would not have formed an 

 insuperable difficulty, unless it could likewise have 

 been shown that at the same period the species were 

 suddenly and simultaneously developed in other 

 quarters of the world. It is almost superfluous to 

 remark that hardly any fossil-fish are known from south 

 of the equator ; and by running through Pictet's Palseon- 

 tology it will be seen that very few species are known 

 from several formations in Europe. Some few families 

 of fish now have a confined range ; the teleostean fishes 

 might formerly have had a similarly confined range, 

 and after having been largely developed in some one 

 sea, have spread widely. Nor have we any right to 

 suppose that the seas of the world have always been 



